Genetic engineering of plants, which entails the isolation and manipulation of genetic material (usually in the form of DNA or RNA) and the subsequent introduction of that genetic material into a plant or plant cells, offers considerable promise to modern agriculture and plant breeding. Increased crop food values, higher yields, feed value, reduced production costs, pest resistance, stress tolerance, drought resistance, the production of pharmaceuticals, chemicals and biological molecules as well as other beneficial traits are all potentially achievable through genetic engineering techniques. Once a gene has been identified, cloned, and engineered, it is still necessary to introduce it into a plant of interest in such a manner that the resulting plant is both fertile and capable of passing the gene on to its progeny.
A variety of methods have been developed and are currently available for the transformation of various plants and plant cells with DNA. Generally, these plants have been dicotyledonous, and some success has been reported with certain of the monocotyledonous cereals. However, some species have heretofore proven untransformable by any method. Thus, previous to this discovery, no technology had been developed which would permit the production of stably transformed Zea mays plants in which the introduced recombinant DNA is transmitted through at least one complete sexual cycle. This failure in the art is well documented in the literature and has been discussed in a number of recent reviews (I. Potrykus, Trends in Biotechnology, 7, 269 (1989); K. Weising et al., Ann. Rev. of Genetics, 22, 421 (1988); F. Cocking et al., Science, 236, 1259 (1987)).
Some of the techniques attempted, or proposed, for introducing DNA into corn cells include electroporation, microinjection, microprojectile bombardment, liposome fusion, Agrobacterium-mediated transfer, macroinjection, and exposure to naked DNA in solution.
For example, J. DeWet et al., Experimental Manipulation of Ovule Tissue, G. Chapman et al., eds., Longman, Inc., New York (1985) at pages 197-269 and Y. Ohta, PNAS USA, 83, 715 (1986) reported the introduction of DNA into maize by mixing pollen grains with DNA solutions, and applying the pollen to maize silks. In these papers, there is no molecular data confirming the introduction of the exogenous DNA into the corn cells. Arntzen et al. in published European Patent Application No. 275,069 describe the incubation of DNA with maize pollen followed by pollination of maize ears and formation of seeds. The plants derived from these seeds were reported to contain the introduced DNA, but there is no suggestion that the introduced DNA was transmitted through a complete sexual cycle.
A. Graves et al., Plant Mol. Biol., 3, 43 (1986) reported Agrobacterium-mediated transformation of Zea mays seedlings. The evidence was based upon assays which can sometimes be unreliable. To date, there have been no further reported successes with pollen and Agrobacterium-mediated transfer techniques.
Microprojectile bombardment has been reported to yield transformed corn cells. The technique is disclosed in Sanford et al., Part. Sci. & Techn., 5, 27 (1987) as well as in published European patent application number 331,855 of J. C. Sanford et al. which is based upon U.S. Ser. No. 07/161,807, filed Feb. 29, 1988. Klein et al., Plant Physiol., 91, 440 (1989) describe production of transformed corn cells, using microprojectile bombardment. However, the cells used were not capable of regeneration into plants. Thus, no protocols have been published describing the introduction of DNA by a bombardment technique into cultures of regenerable maize cells of any type. No stable introduction of a gene has been reported that results from bombardment of maize callus followed by regeneration fertile plants and transmission of the introduced gene through at least one sexual cycle. D. McCabe et al., in published European patent application No. 270,356, disclose the bombardment of maize pollen with DNA, the application of the pollen to silks, and the formation of seeds which reportedly contain the exogenous DNA. However, there is no evidence that the DNA was transmitted through a complete sexual cycle, and no further results have been reported by this group.
Electroporation of corn protoplasts has been reported to result in transformed cells by M. E. Fromm et al., Nature, 319, 791 (1986) although these cells did not provide regenerated plants. Electroporation of corn protoplasts was also reported by C. Rhodes et al., Science, 240, 204 (1988). Although the recipient cells were transformed and, in the latter case, were able to regenerate into plants, the plants themselves were sterile. In addition, methods for the production of the cell line used by Rhodes et al. were not reproducible.
A further stumbling block to the successful production of fertile transgenic maize plants has been in selecting those few transformants in such a manner that neither the regeneration capacity (in the case of protoplasts or cell cultures) nor the fertility of the transformants are destroyed. Due to the generally low level of transformants produced by a transformation technique, some sort of selection procedure is often necessary. However, selection generally entails the use of some toxic agent, e.g., a herbicide or antibiotic, which may be detrimental to either the regenerability or the resultant plant fertility.
On the other hand, it has been known that untransformed corn protoplasts, cultured cells and callus at least can be regenerated to form mature plants and that the resulting plants are often fertile. For example, R. D. Shillito et al., Bio/Technology, 7, 581 (1989), and L. M. Prioli et al., Bio/Technology, 7, 589 (1989) discuss methods for producing protoplasts from cell cultures and recovering fertile plants therefrom. C. A. Rhodes et al., Bio/Technology, 6, 56 (1988) disclose attempts to regenerate maize plants from protoplasts isolated from embryogenic maize cell cultures.
However, it has not been possible for the art worker to determine which maize tissues or cultures are appropriate recipients for exogenous DNA, e.g., contain a useful number of cells which are receptive to and which will stably integrate the exogenous DNA, and at the same time be a part of the germline, i.e., part of the cell lineage that leads to the next plant generation.
The art is thus faced with a dilemma. While certain transformation techniques have been proposed or reported to produce transformed maize cells and certain cells and tissues have been proposed to be potential recipients due to their ability to regenerate plants, the art has failed to find a combination of techniques which would successfully produce transformed maize plants able to transmit the introduced DNA through one complete sexual cycle.
It is thus an object of the present invention to produce fertile, stably-transgenic, Zea mays plants and seeds which transmit the introduced gene to progeny. It is a further object to produce such stably-transgenic plants and seeds by a particle bombardment and a selection process which results in a high level of viability for at least a portion of the transformed cells. It is a further object to produce fertile, stably-transgenic plants of other graminaceous cereals besides maize.